Many roofing systems use only one layer of fabric embedded in roofing tars and chemical coatings such as butyl rubber, urethanes and acrylics. These fabrics are usually made of cotton, glass or polyester fiber.
Roofing membranes employing cotton fiber fabrics as the reinforcing component suffer from the disadvantage that cotton is subject to decomposition. Although fabrics employing glass fiber are not subject to this defect, they have other disadvantages such that they have no elasticity and hence tend to split when thermal expansion of the roof occurs. Glass fibers exhibit the phenomenon of capillary attraction or "wicking" as it is known in the art, and as a result, take up water which shortens the life of the roofing membrane as an effective waterproof system. Polyester fiber is the best choice for flexibility; but roofs with only one layer suffer from leakage at the seams, and some light can penetrate single coatings sufficiently to cause degradation of the polyester fabric.
The type of waterproofing material is very important to the effective life of the roofing membrane. Roofing membranes employing tars, asphalts, and bituminous pitches are subject to cold weather embrittlement which results in cracking. These coatings do not effectively resist the damaging ultraviolet rays of the sun. Roofing membranes made with some urethane coatings also have short effective lives due to the combined effects of water and solar ultraviolet radiation. Butyl rubber coatings have superior flexibility, but ultraviolet rays eventually cause embrittlement and subsequent splitting. Acrylic formulated polymers have superior weathering properties which retain flexibility and remain waterproof over exceptionally long weathering periods. Test results reveal that acrylic latex polymers exhibit no cracking, embrittlement or significant discoloration after six thousand hours of accelerated aging.
Fully adhered roofing membranes experience the problem of cracking due to thermal expansion and contraction of the roof. That is, at a given point on the roof, the thermal expansion of the roof may exceed the elastic limits of the fully adhered membrane, causing cracking. A spot-anchored membrane is preferable, as it spreads the force due to thermal expansion over a greater area, thus reducing the stress and resultant risk of cracking.